Device and method for determining frequency and amplitude of an oscillating structure, especially for measuring acceleration or rotational rates

ABSTRACT

A device for determining the frequency and/or the amplitude of a vibrating structure includes a vibrating element ( 2 ) and a pair of position sensors ( 10, 11 ) for the determination of the deflection of the vibrating element ( 2 ). The position sensors ( 10, 11 ) are arranged such that their measurements during a half-wave of vibration exceed and/or are less than one another. A comparator compares the measurements of the two position sensors ( 10, 11 ) to determine a threshold value U s  for the half-wave of the vibration at which their measurements are equal. A device is used for determining the duration during which the measurement of one of the two position sensors ( 10, 11 ) exceeds or is less than the threshold value U s . The position sensors ( 10, 11 ) can be capacitors whose electrodes are arranged in a step-like manner. The determination of the amplitude of the vibration is, carried out independent of a potential parallel shift of the movable element ( 2 ), such that there is no distortion of the measurement result.

BACKGROUND OF THE INVENTION

The present invention relates to a device and a method for determiningthe frequency and/or amplitude of a vibrating structure, particularlyfor the measurement of accelerations or angular rates.

Vibrating structures have various engineering applications, For example,they are used as acceleration and/or angular rate sensors, wheremovement quantities can be determined from the vibrationcharacteristics.

The U.S. Pat. No. document 4,598,585 describes an angular rate sensorwhere a mass is supported around two axes (x-axis, y-axis) that areperpendicular to one another in a vibrating manner. To determine theangular rate around the z-axis that runs vertically to the x- andy-axis, the element is excited to a periodic vibration around they-axis. The Coriolis forces during the rotation cause an additionalvibration of the element around the x-axis, whereby the amplitude is ameasure for the angular rate. Such an angular rate sensor with dualcardanic suspension is shown in FIG. 3.

Additionally, acceleration sensors are known that carry out anoscillating movement, whereby the frequency is a measure foracceleration acting in a certain direction.

However, the known vibrating structures have the disadvantage that nodistinction can be made between a change in vibration amplitude and aparallel shift of the vibrating element. For example, distance changesof the suspension of the moveable element may occur that falsify themeasured quantity. When sensing the position or the deflection of thevibrating element with a capacitor, the smallest changes in the distanceof the capacitor plates can overlay the actual vibration amplitude suchthat exact measurement results are no longer possible. This appliesparticularly in cases where the interference or the parallel shift is inthe same frequency range as the excited vibration.

It is, therefore, the objective of the present invention to create adevice for the determination of the frequency and/or amplitude of avibrating structure and to provide a method for measuring the vibrationamplitude with a high degree of accuracy, whereby interferences, e.g.,due to a parallel shift or a distortion of the structure can besuppressed effectively.

SUMMARY OF THE INVENTION

The device according to the invention for the determination of thefrequency and/or the amplitude of a vibrating structure is particularlysuited to measure the acceleration or angular velocity and has a movableelement that can be excited in vibration, a pair of position sensors todetermine the deflection of the movable element, that are arranged suchthat during a half-wave of the vibration their measurements exceedand/or fall short of each other, a circuit for comparing themeasurements of the two position sensors for determining from themeasurements a threshold value for the half-wave of the vibration, and adevice for determining the duration during which the measurement of oneof the two position sensors exceeds and/or falls short of the thresholdvalue.

The special arrangement of the position sensors and the comparison ofthe measurements with threshold value determination in one half-wave ofthe vibration has the effect that vibration amplitude is determinedaccurately even with a measurement interference due to a parallel shiftof the vibrating element. Errors in determining the amplitude areavoided.

Advantageously, the device comprises another pair of distance sensors todetermine a second threshold value for the second half-wave of thevibration. These distance sensors are preferably arranged such that thethreshold values in the positive and in the negative half-wave have thesame value. Especially in a mechanical structure that vibrates around adefined axis, an acceleration that acts perpendicular to the vibrationaxis A will no longer result in errors in the amplitude measurement.

Advantageously, the position sensors are capacitors, which results in acost-effective design. However, they may also be made of opticalelements or similar sensors known to the professional for position ordistance measurements.

Preferably, one element of a pair of position sensors is arranged highersuch that the threshold is set at a defined deflection of the movableelement. The threshold values can, for example be activated throughmechanical stages, or step-like arrangement of electrodes at a vibrationelement or on a surface opposite the element. Preferably, the positionsensors are arranged at different distances from the rotating axis ofthe movable element.

The method subject to the invention for determining the frequency and/oramplitude of a vibrating structure comprises the steps: Determining athreshold value in the positive and/or negative half-wave of avibration, determining the duration for exceeding and/or falling shortof the threshold value during a vibration, and determining the frequencyand/or amplitude from the duration of exceeding and/or falling short ofthe threshold value. In this manner, the vibration amplitude can bedetermined regardless of the distance between the vibrating element anda fixed element and the frequency of the vibration can be determined aswell.

Advantageously, a threshold is determined for both the positive and thenegative half-wave of the vibration and the frequency and/or theamplitude is determined from the duration of exceeding and/or fallingshort of the two threshold values. The threshold values in the positiveand the negative half-waves may have the same value. Especially, theycan be formed by mechanical steps.

In the following, the present invention is described using a preferredembodiment. In the Figures,

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 shows schematically a sectional view of a preferred embodiment ofthe device according to the invention.

FIG. 2 shows a diagram for explaining the vibration and thecharacteristic quantities that according to the method of the inventionare used to measure the amplitude and frequency.

FIG. 3 shows a known angular rate sensor with a vibration-capablestructure.

The position sensors 10, 11 are connected with a comparator thatcompares the measurements of the two sensors 10, 11 and determines athreshold value U_(s), where the same measurement value applies to bothsensors 10, 11. When using a device for measuring the time, thevibration amplitude A_(s) and the frequency f_(s) of the vibration canbe determined from the threshold value U_(s) and the signal of a sensor,as will be described in greater detail.

In the embodiment shown here, the position sensors 10, 11 are formed bycapacitors. In the preferred embodiment, a pair of position sensors 10,11 and 20, 21 or capacitors, is located on either side of the rotatingaxis A of the rocker 2. One electrode 11 a, 21 a each of a capacitorpair 10, 11 or 20, 21 is attached to the rocker 2 in a raised manner.This results in the fact that the electrodes 11 a, 21 a that areattached in a raised manner are closer to the counter electrode in theidle position of the rocker, that is, at a deflection angle of φ=0, thanthe two other electrodes 10 a, 20 a. In the present case, this isrealized in that the surface 8 of the rocker 2, where the electrodes 10a, 11 a, 20 a, 21 a are arranged, has a step-like design. To realize thedevice subject to the invention, it is, however, also possible toprovide the opposite plate 1 with electrodes at different levels insteadof the rocker 2. Regardless of the respective embodiment or the concretedesign of the position sensors 10, 11 or 20, 21, the important fact isthat with a defined deflection of the rocker 2 by the angle φ, twoposition sensors 10, 11 show the same reading, while their values differat other angles.

The second pair 20, 21 of position sensors or capacitors has the resultthat at a certain deflection in either direction, one pair of positionsensors will display the same measurement value. This value defines thethreshold value U_(s) of the respective half-wave of the vibration thatwill be exceeded by one of the measurement signals with a deflectionthat goes beyond the angle +φ or −φ.

The height of the raise or the overheight of one of the electrodes 11 a,21 a of a pair of capacitors 10, 11 or 20, 21 determines the thresholdvalue U_(s). Thus, the threshold value U_(s) can be set differentlycorresponding to the requirements.

FIG. 2 will be used to explain how the measurement signals of theposition sensors 10, 11, 20, 21 or the capacitors of the vibrationamplitude determine the vibration amplitude regardless of the distance dbetween the rocker 2 and the plate 1. The signal amplitude A_(s), forexample, is measured using the position sensor 10, which in FIG. 1 islocated at the right side of rocker 2 in its outer region. At adeflection from the zero position by the angle φ, the right part of therocker 2 approaches the opposite plate 1, whereby the two positionsensors 10 and 11 will exhibit the same value at a defined angle φ,which will be designated as the threshold value +U_(s) by thecomparator. With a continued deflection in the positive direction, themeasurement signal of the position sensor 10 will exceed the positivethreshold value or the threshold voltage +U_(s), will then reach itsmaximum and then fall short of the threshold voltage +U_(s) during themovement of the rocker 2 in the counter direction. The duration T_(p)between exceeding and falling short of the threshold voltage +U_(s) ismeasured. Then, the rocker 2 will go through its zero position and adeflection in the counter direction will occur, such that with anegative deflection by the angle −φ, the two position sensors 20, 21 inthe left part of the rocker 2 will display the same value from which thethreshold value −U_(s) is formed for the second or negative half-wave.With a continued deflection in this direction, the measurement signalwill exceed this threshold value during a duration T_(N).

The vibration amplitude can now be determined from the threshold voltageU_(s), the frequency f_(s) and the duration T_(p) of exceeding thethreshold value +U_(s), according to the equation $\begin{matrix}{A_{s} = \frac{U_{s}}{{COS}\quad \left( {T_{P}*\pi*f_{s}} \right)}} & (1)\end{matrix}$

The frequency f_(s) can be determined from two successive thresholdexceedings in one direction requiring in this case only one pair ofposition sensors to determine the threshold value of one half-wave.

With two pairs of position sensors 10, 11 and 20, 21, where each pair islocated on one side of the rotation axis A, the frequency f_(s) orf_(s′π)can be determined from the duration T_(fp) or T_(fn) according tothe equation

f _(s)=½*T _(fp)  (2)

or

f _(s′)=½*T _(fn)  (3),

where T_(fp) is the duration between exceeding the threshold value ofthe positive half-wave and exceeding the threshold value of the negativehalf-wave. T_(fn) is the duration between falling short of the thresholdvalue of the positive half-wave and falling short of the threshold valueof the negative half-wave.

The step-like signals S_(p) and S_(n) are formed from the durationT_(p), where the amplitude is above the threshold voltage +U_(s), andthe duration T_(n), where the amplitude is below the negative thresholdvoltage −U_(s), and then processed in order to calculate the amplitudeand/or the frequency of the vibration regardless of the distance dbetween the rocker 2 and the plate 1.

The structure or device is produced micro-mechanically, which allows fora cost-effective series production with a small design. The inventionhas the result that no errors occur in the amplitude measurement whenthere is a parallel shift of the movable elements 2 that may be caused,for example, by an acceleration acting upon the structure in a directionperpendicular to the rotating axis A.

The device subject to the invention may be used, for example, inconnection with the known angular rate sensor shown in FIG. 3. There, aninner element 31 is supported in a pivotal fashion around the x-axis ina frame 30 that rotates around the y-axis. The device subject to theinvention can be used to measure the deflection of the inner element 31or the frame 30 without an error occurring in the amplitude measurementdue to a parallel shift of the frame 30 or the element 31 and theresultant change in the measurements of individual position sensors. Torealize this, pairs of position sensors are arranged at the frame 30and/or at the inner element 31 to measure the deflection and determinethe threshold value. Amplitude and frequency can then be determined fromthe measurement values as described above, without interferences througha shift influencing the values for the amplitude or the frequency.

Aside from this example, numerous other applications are possible wherea deflection of elements supported in a rotating or pivotal manner isused to measure physical quantities. Regardless of the respectiveapplication, significant is the fact the interferences that occur whendetermining the frequency or the amplitude due to the parallel shift ordistortion can be avoided.

What is claimed is:
 1. Apparatus for determining frequency and/oramplitude of a vibrating structure adapted for the measurement ofacceleration and angular velocity comprising: a fixed element, a movableelement facing said fixed element at a distance therefrom, said movableelement being excited in periodic vibration about an axis of vibrationin a plane of the movable element for undergoing angular vibratingmovement around said axis of vibration towards and away from said fixedelement, first and second position sensors spaced at different distancesfrom said axis of vibration for measuring distance of said movableelement from said fixed element as said movable element undergoesvibration around said axis, the first and second sensors respectivelyincluding sensor elements supported at different levels on said movableelement so that as the movable element vibrates the sensor elements ofthe first and second sensors are at equal distances from the fixedelement for one particular angular position of the movable element ineach half wave of the vibration thereof and are at different distancesfrom the fixed element at all other angular positions of the movableelement during said vibration thereof, a comparator for comparingdistance measurements of the sensor elements to determine a thresholdvalue at which the distance measurements are equal and the movableelement is at said one angular position in each half wave of vibration,and a device for determining amplitude and/or frequency of vibration ofthe movable element from said threshold value, independently of thedistance of the movable element from the fixed element.
 2. The apparatusof claim 1, wherein said first and second sensors are located on oneside of said axis.
 3. The apparatus of claim 2, comprising third andfourth position sensors located on an opposite side of said axis ofvibration, the first and second position sensors determining thethreshold value in one half wave of vibration of the movable element andthe third and fourth sensors determining the threshold value in asubsequent half wave of vibration.
 4. The apparatus of claim 3, whereinthe first and second position sensors and the third and fourth positionsensors are arranged such that the threshold values in the half waves ofvibration have the same value.
 5. The apparatus of claim 3, wherein theposition sensors comprise capacitors or optical elements.
 6. Theapparatus of claim 2, wherein the sensor element of one of first andsecond sensors is supported at an elevated level on a surface of themovable element such that the threshold value is obtained for a definedparticular angular position of the movable element.
 7. The apparatus ofclaim 3, wherein each of the position sensors comprises a pair ofelectrodes respectively arranged on a surface of the movable element andon an opposite surface of the fixed element.
 8. The apparatus of claim7, wherein the surface of the movable element has a step, one saidelectrode of one of the sensors being located on said step.
 9. Theapparatus of claim 1, wherein said sensor elements of said positionsensors comprise a fixed electrode on said movable element and anopposite fixed electrode on said fixed element.
 10. The apparatus ofclaim 1, wherein the device for determining amplitude and/or frequencyof the moving body determines amplitude A_(s) of the movable elementfrom the equation$A_{s} = \frac{U_{s}}{{COS}\quad \left( {T_{P}\quad \pi \quad f_{s}} \right)}$

wherein U_(s) is amplitude of the threshold value, T_(p) is a timeinterval between successive threshold values in a half-wave of vibrationand f_(s) is the frequency of vibration.
 11. The apparatus of claim 1,wherein said fixed element and said movable element are in the form ofplates.
 12. A method for determining frequency and/or amplitude of avibrating structure adapted for measurement of acceleration and angularvelocity comprising the steps of: vibrating a plate element in periodicvibration about an axis in a plane of the plate towards and away from afixed datum, supporting two position sensors on the plate element formeasuring distance of the vibrating plate with respect to the fixeddatum, arranging the position sensors on the plate to obtain a thresholdvalue (U_(s)) during a half-wave of vibration of the plate at which thedistances measured by the two position sensors from the fixed datum areequal, determining time duration (T_(p)) in the half-wave of thevibration between successive threshold values, and determining frequencyor amplitude or both of vibration of the plate from the threshold valueU_(s) and time duration T_(p) independently of spacing between themovable plate and the fixed datum.
 13. The method of claim 12, whereinduring a positive half-wave of vibration of the vibrating plate, thedistance measured by the position sensors exceeds the distance measuredat the threshold value whereas during a negative half-wave of vibrationof the plate, the distance measured by the position sensors is less thanthe distance measured at the threshold value.
 14. The method of claim13, wherein for the positive and the negative half-waves of vibrationone threshold value is measured for the positive half wave and a secondthreshold value is measured for the negative half wave, and determiningfrom the respective periods of duration between the two thresholds, thefrequency of the vibration.
 15. The method of claim 14, wherein thethreshold values (U_(s)) in the positive and negative half-waves areequal.
 16. The method of claim 12, comprising determining amplitudeA_(s) of vibration of the movable element from the equation$A_{s} = \frac{U_{s}}{{COS}\quad \left( {T_{P}\quad \pi \quad f_{s}} \right)}$

wherein U_(s) is amplitude of the threshold value, T_(p) is the timeinterval between successive threshold values in a half-wave of vibrationand f_(s) is the frequency of vibration.
 17. The method of claim 12,wherein the threshold value (U_(s)) is obtained by supporting theposition sensors at different elevations on the vibrating plate.